Sunny Days Protect Against Flu

Vitamin D supplementation is cheap. Walking in sunlight is even cheaper. I’ve been doing more of both since the beginnings of the pandemic. Slusky and Zekhauser add to the evidence:

Sunlight, likely operating through the well-established channel of producing vitamin D, has the potential to play a significant role in reducing flu incidence. A recent meta-analysis of 25 randomized controlled trials of vitamin D supplementation (Martineau et al. 2017) demonstrated significant benefits of such supplements for reducing the likelihood that an individual will contract an acute upper respiratory infection. The current study considers sunlight as an alternate, natural path through which humans can and do secure vitamin D. This study’s findings complement and reinforce the Martineau et al. findings.

Our major result is that incremental sunlight in the late summer and early fall has the potential to reduce the incidence of influenza. Sunlight had a dramatic effect in 2009, when sunlight was well below average at the national level, and the flu came early. Our result is potentially relevant not just to the current COVID-19 pandemic, but also to a future outlier H1N1 pandemic. The threat is there; some H1N1 viruses already exist in animals (Sun et al. 2020). One must be cautious, though, with generalizations, given the unique economic circumstances (e.g., a 25-year high unemployment rate) in the fall of 2009.

A remaining question is whether sunlight matters more broadly for flu, or whether it is unique to H1N1. While we lack a counterfactual of an early flu from a different strain, we do have two pieces of evidence to suggest that the effect is broader than just H1N1. First, as described throughout the paper, the Martineau et al. study about the relationship between Vitamin D and upper respiratory infections are not specific to H1N1. Second, with granular, county level data, we do see strongly statistically significant negative effects of fall sunlight on influenza for years other than 2009 (see Columns (2) and (3) of Panel of Table 7). Therefore, apart from its methodological contributions, this study reinforces the long-held assertion that vitamin D protects against acute upper respiratory infections. One can secure vitamin D through supplements, or through a walk outdoors, particularly on a day when the sun shines brightly. When most walk, herd protection provides benefit to all.

The Scots are giving out free vitamin D to people stuck indoors. My view is that Vitamin D supplementation is worthwhile but where and when possible the sunlight approach is better as the effect may work through mechanisms beyond vitamin D.

Coincidentally neuroscience says this is one of the happiest songs ever.

Hat tip: The sunny Kevin Lewis.

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